This invention relates generally to the art of drilling fluids used in oilfield application. More particularly it relates to dissolution of filtercakes made by drilling fluids and methods to dissolve filtercakes in a well from which oil and/or gas can be produced. Most particularly it relates to a multifunction fluid for the efficient dissolution of filtercakes.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In order to produce hydrocarbon fluids from subterranean formations, a borehole is drilled from the surface down into the desired formations. During the drilling step, reservoir drilling fluid (RDF) is circulated within the drilling equipment first, to cool down and clean the drill bit, secondly, to remove the drill cuttings out of the wellbore, thirdly, to reduce friction between the drilling string and the sides of the borehole, and fourthly to form a filtercake in order to prevent fluid leak off into the formation. The driving force for the formation of the filtercake is the higher wellbore pressure applied to maintain the borehole stability. This filtercake restricts the inflow of reservoir fluids into the well during the drilling process and placement of the completion. If the filtercake damage that is created during the drilling process is not removed prior to or during completion of the well, a range of issues can arise when the well is put on production (i.e. completion equipment failures and impaired reservoir productivity). The solids in the RDF are sized such that they form an efficient bridge across the pores of the formation rock as it is being drilled. As the solids in the RDF develop bridges across the exposed pores (pore throats) of the reservoir, the polymeric fluid loss material (i.e. starch) from the RDF is co-deposited within the interstices of the solid bridging particles, thus sealing off the reservoir from the wellbore. Analyses of a family of filtercakes show that the filtercakes comprise roughly 55-61% calcium carbonate, 17-18% polymers, and 16-20% drill solids (which are most often aluminosilicates/clays).
Calcium carbonate (acid-soluble) and polymers (soluble in oxidative or enzyme breakers) are relatively simple to dissolve from the RDF filtercakes. However, the most difficult portion of the filtercake to dissolve remains the insoluble drill solid particulates. There are several services/fluids currently executed to dissolve filtercakes to restore productivity/injectivity between the formation and wellbore. Currently, Schlumberger uses a fluid known as MudSOLV (Trade Mark from Schlumberger), which uses an array of chemicals targeting specifically two of the three components of the water-based RDF filtercake: chelants or acids are used to dissolve the calcite component, enzymes or oxidizers are used to degrade the polymer component.
However, even MudSOLV benefits from a large commercial success and technical advance in the field, major limitation is its failure to dissolve the third component of the water-based RDF filtercake i.e. the drilling solids and clays. By comparison, mud acid is well known to efficiently dissolve aluminosilicate materials. However, mud acid treatments (including in filtercake dissolution) involve a number of stages of functional fluids used in sequence to dissolve the various filtercake components: an acid preflush for calcite dissolution followed by the mud acid (HCl/HF) fluid, which dissolves the clay particulates. However, mud acid is highly reactive and is difficult to inhibit the reaction kinetics thereof. As a result, aggressive mud acid types of treatments have been known to pinhole through a filtercake rapidly, creating massive fluid losses to the formation only through the pinhole (and inefficiently dissolving the remainder of the filtercake).
Some prior arts intended to fix some of the disclosed drawbacks. U.S. Pat. No. 7,134,496 from Jones et al. addresses technology surrounding the methods of use of hydrocarbon-external, acid-internal emulsions to dissolve filtercakes deposited through the use of oil-based muds. The acidizing fluids described herewith include chelating agents generally described as polyaminopolycarboxylic acids. However, the ability to dissolve simultaneously calcite and clays is not described. SPE publications “First Application of Novel Microemulsion Technology for Sand Control Remediation Operations—A Successful Case History From the Rosa Field, a Deepwater Development Project in Angola,” from Lavoix et al. and “NAF Filtercake Removal Using Microemulsion Technology” from Quintero et al, disclose acid/surfactant/hydrocarbon emulsions used in the dissolution of oil-based filtercakes. However, again, these acidizing fluids lack the ability to dissolve appreciable amounts of aluminosilicate materials (which can be a large portion of the overall filtercake composition). U.S. Pat. No. 6,978,838 from Parlar et al. describes the sequential execution of a solution for polymer dissolution, a high-rate displacement of that fluid (to clean the wellbore), followed by a solution of acid and/or chelating agent to dissolve the calcite portions of the filtercake. However, Parlar et al. describes specifically the use of acids that do not significantly react with siliceous formations.
Therefore, there is a need for the development of an alternative solution first, that is capable of dissolving all three components of a filtercake, secondly that has retarded reaction kinetics (to prevent pinholing), and lastly that minimizes the number of stages of fluid in a mud acid treatment, and in the case of injector wells, minimizes the need to flowback the spent fluid to the surface and ensures that the spent fluid can be squeezed into the formation directly after treatment.